(349d) Crystal Shape Enhancement: a Processing Solution to a Product Problem

Crystal shape is an important measure of product quality with direct implications for a wide variety of sectors, including, the pharmaceutical, food, home & personal care, and dye industries. The shape affects downstream processes such as filtering, washing and drying as well as material properties such as mechanical strength, bioavailability, catalytic activity, and surface functionality. For example, needle-like crystals present difficulties in washing and drying, but can be favored for systems where a large surface area to volume ratio is required. Also, particular crystal surfaces can either be specifically desired or undesired due to the surface characteristics. For a crystal with a single face of particularly high catalytic activity, the area of such a face should be optimally maximized; however, a crystal such as adipic acid where one crystal face's hydrophobicity makes bulk transport difficult, the area of that face should be minimized.

Most industrial crystallizations take place in solution; therefore, existing efforts in engineering enhanced crystal shapes rely directly upon changing the chemical nature of the crystallization. Different solvents lead to different crystal shapes; mixed solvents can lead to even more flexibility, and additives, both tailor made and surfactants, have been used to chemically alter the shapes that crystals can obtain. Here we present a novel processing technique that could lead to enhanced crystal shapes. Such a non-chemical route to crystal shape enhancement has the benefit of not requiring additional impurities or separations but rather utilizes the existing solute-solvent solution.

We have combined recently developed crystal shape evolution models for both growth¹ and dissolution² in order to predict the result of cycling a faceted crystal through repetitive steps of growth and dissolution. Utilizing this combined model we demonstrate that a change in crystal shape occurs as a result of cycling when relative growth and dissolution rates are anisotropic or when any of the growth faces disappear during the dissolution step. We have quantified the magnitude and direction of the shape enhancement as a function of the relative growth and dissolution rates. While some crystal shapes become more bulky as the process is cycled, others become more plate-like or needle-like during cycling. Key modeling concepts will be presented as well as results for both an illustrative crystal system as well as predictions for succinic acid undergoing growth/dissolution cycling in water. Finally, new physical embodiments of crystallizer equipment for such a cycling process will be discussed.

References

1. Zhang Y, Sizemore J, Doherty MF. Shape Evolution of 3-Dimensional Faceted Crystals. AICHE J. 2006; 52:1906-1915.

2. Snyder RC, Doherty MF. Faceted Crystal Shape Evolution During Dissolution or Growth. Submitted to AICHE J.: April 2006.